With the advent of the worldwide energy crisis and enhancement of people's environmental protection awareness, water hydraulic technologies have been found to have advantages over oil hydraulic systems in many application fields (e.g., during underwater operations, or in buoyancy adjustment of manned submersibles) owing to the special physicochemical properties of the water medium. Accordingly, water hydraulic technologies have experienced a rapid development.
However, because viscosity of the water is only about 1/30˜ 1/50 of that of the commonly used hydraulic oil, it is less apt to form a water film and also has poor lubricity. Meanwhile, because of the strongly corrosive nature of the water and particularly the sea water, selection of materials used in water hydraulic systems is limited. This imposes great difficulties in design of friction couplings of water hydraulic elements. For these reasons, in contrast to oil hydraulic pumps, matured axial water hydraulic pumps are mostly designed to work at a medium or high pressure, which is usually 12 MPa˜21 MPa.
A fully water-lubricated sea-water/fresh-water pump in the prior art adopts a plate valve for flow distribution, and has a flow rate of 10 L/min˜170 L/min, a pressure of 14 MPa˜16 MPa and an overall efficiency of higher than 82%. A schematic structural view of such a pump is shown in FIG. 1. As shown, this fully water-lubricated sea-water/fresh-water pump features a compact structure, full water lubrication of the friction couplings, and easy maintenance. Unfortunately, such a pump also suffers from the following shortcomings:
1. The maximum working pressure is 16 MPa, which cannot satisfy the needs in particular applications, for example, in the buoyancy adjusting system of a large-depth (i.e., the submerging depth exceeds 3,000 meters) manned submersible.
2. Distributing the flow by use of a valve plate is, on one hand, unsuitable for open systems because the valve plate is sensitive to pollutants, and on the other hand, makes it difficult to ensure the volumetric efficiency when the water is high-pressurized.
3. As a mechanism comprised of a swash plate and a shoe is used, a large lateral force is applied by the plunger to the cylinder. Hence, serious abrasion will occur to the friction coupling after the water is high-pressurized.
For water hydraulic pumps of higher pressures, a crank-shaft and connecting-rod structure is usually adopted, and a mineral oil lubricated structure with the oil and the water being separated is used for the primary frictional coupling. Water hydraulic pumps of this structure are one of the kinds that are the most widely used around the world, an example of which is a triple plunger pump in the prior art whose pressure range is 55 MPa˜275 MPa. However, the water hydraulic pumps of this structure mainly have the following problems:
1) They have a low rotation speed (100 rev/min˜500 rev/min), a bulky volume, and a small power-to-weight ratio. If the rotation speed is increased to decrease the volume of the pump, the seal between the water cavity and the lubricant oil cavity would be overheated and even fail, which is particularly the case under high-pressure conditions. Meanwhile, the temperature of the oil in the closed lubricant oil cavity may also increase due to poor heat dissipation to cause degradation of the oil.
2) Lubricant oil must be used for lubrication, which tends to cause oil pollution; furthermore, when the water hydraulic pumps are used in deep sea environments, an additional pressure compensation device must be used, which makes the whole structure very complex.